I have a recipe for a berry buttermilk cake with a struesel topping. my question is why does the struesel topping sink into the cake

According to this article in Cooks Illustrated https://www.cooksillustrated.com/articles/577-rethinking-coffee-cak
it’s not so much that the streusel sinks, but that the batter rises up around the edges of the streusel and swallows it. They solved the problem by adding more flour to the cake batter, increasing it from 1 1/2 cups to 1 2/3 cups. I hope this helps.